Chopped low power magnetic-field detector with hysteresis memory

A magnetic-field detector for detecting an ambient magnetic field that is greater than a predetermined field strength, comprising:

• (a) a magnetic-field-to-voltage transducer;
• (b) a transducer-voltage amplifier having an input connected to said transducer;
• (c) a zero-crossing comparator having an input connected to the output of said amplifier for producing a binary output signal of one kind when the voltage at said comparator input is of one polarity and producing an output signal of the other kind when the voltage at said comparator input is of the other polarity;
• (d) a free-running clock means connected to said transducer for producing a binary output clock signal alternating between one and another levels and for energizing said transducer when said clock signal is at the one level and de-energizing said transducer when the clock signal is at the another level;
• (e) a clockable flip flop connected to said clock and having an input connected to the output of said zero-crossing comparator, for only at an instant, when the comparator output binary signal is of the one kind and when simultaneously the clock signal is changing from the one level to the another level, producing at said Q output a binary signal of one kind;
• and
• f) a positive-feedback hysteresis circuit means connected from the output of said flip flop back to said transducer-voltage amplifier for adding a bias to the amplifier output signal at said comparator input only when the output of said flip flop is the one kind, and transforming said zero-crossing comparator into a comparator having hysteresis with memory wherein the magnitude of said bias establishes said predetermined magnetic field strength above which detection of the field is effected.

A method for detecting a magnetic-field of greater than a predetermined field strength, comprising:

• (a) providing a magnetic-field-to-voltage transducer;
• (b) providing a zero-crossing comparator having a binary output signal composed of one and another voltage levels corresponding respectively to a positive and a negative comparator input signal;
• (c) periodically clocking and enabling said transducer for only a first portion of each clock period;
• (d) remembering, during the remaining portion of said each clock period, the binary voltage level of the comparator output signal existing for an interval in the first clock period portion;
• (e) adding to the transducer output signal a positive predetermined bias voltage only when the remembered binary comparator output voltage level is the one level to produce a sum signal;
• and
• (f) applying the sum signal to the input of said comparator, whereby the remembered binary voltages level serves as an indication whether or not there exists an ambient magnetic field of magnitude greater than a predetermined field strength, said predetermined field strength being a fixed function of said bias voltage.